FIG. 6 is a schematic block diagram showing a conventional air separation apparatus.
In the past, when oxygen, argon, etc. were produced by low-temperature processing cryogenic separation of air, an air separation apparatus 200 shown in FIG. 6 was used.
As shown in FIG. 6, the air separation apparatus 200 includes an air compressor 201, an air precooler 202, an air purifier 204, a turbine blower 205, a turbine blower aftercooler 206, a turbine 208, a main heat exchanger 211, a low-pressure column 213, a low-pressure column reboiler 214 at the bottom part of the low-pressure column 213, an middle-pressure column 216, a subcooler 218, an argon column 221, and an argon column condenser 222 at the top part of the argon column 221.
When oxygen, nitrogen, argon, etc. were produced using the air separation apparatus 200, oxygen enriched liquefied air, which was withdrawn from the bottom part of the middle-pressure column 216, was vaporized in the argon column condenser 222, and then introduced into the low-pressure column 213 as oxygen enriched air. In the air separation apparatus 200, low-pressure liquefied oxygen at the bottom part of the low-pressure column 213 was reboiled using middle-pressure nitrogen gas at the top part of the middle-pressure column 216.
In addition, when oxygen, nitrogen, argon, etc. were produced using the air separation apparatus 200, it is also possible to withdraw liquefied oxygen (LPLO2) from the bottom part of the low-pressure column 213, or middle-pressure nitrogen gas (MPGN2) and liquefied nitrogen (MPLN2) from the top part of the middle-pressure column 216, in addition to argon gas and liquefied argon (LAR). However, when the flow rate of the liquefied oxygen (LPLO2), the middle-pressure nitrogen gas (MPGN2) or the liquefied nitrogen (MPLN2) increases, the argon recovery decreases.
Moreover, “yield” means the ratio of flow rate of each product relative to the flow rate of feed air supplied in the air separation apparatus 200.
Patent Document 1 discloses an air separation method and an air separation plant which can increase the amount of gaseous oxygen obtained by separation of air by low-temperature distillation using a double column.
Patent Document 1 discloses a method for improving the yield of oxygen by adding a mixing column in addition to a low-pressure column, a middle-pressure column, and an argon column, and overhead gas from the top part of the mixing column is supplied to the bottom reboiler of the low-pressure column.
In addition, Patent Document 1 also discloses that the argon recovery can be maintained or improved even when a flow rate which corresponds to 10 to 15% of feed air is collected as middle-pressure nitrogen gas from the middle-pressure column or a flow rate which corresponds to 10 to 15% of feed air is sent to the low-pressure column as blowing air.
Furthermore, Patent Document 1 discloses that a part of middle-pressure nitrogen gas or a part of the feed air is expanded by a turbine into low-pressure nitrogen or blowing air, coldness is generated, and a liquefied gas product is collected. In other words, even when a certain amount of the liquefied gas product is collected, the argon recovery can be maintained or improved.
Patent Document 2 discloses a technique which can improve the argon recovery. Specifically, Patent Document 2 discloses that oxygen enriched liquefied air withdrawn from the bottom part of a high-pressure column is supplied to a gas-liquid contact part and distilled at low temperatures, gasses having a different oxygen concentration which are separated at the gas-liquid contact part are supplied into a low-pressure column, rectification conditions of the low-pressure column are improved, and thereby the argon recovery is improved.